Conventionally known gas-barrier films include films in which thin films of metal such as aluminum or thin films of inorganic oxides such as silicon oxide and aluminum oxide have been laminated on the surface of a plastic film. In particular, films in which thin films of inorganic oxides such as silicon oxide, aluminum oxide, and a mixture thereof has been laminated have been used widely for food applications because they are transparent and contents can be checked therethrough.
However, pinholes, cracks, and so on are easily formed in such inorganic thin films during a thin film formation process and inorganic thin film layers crack during a processing step, and because of this, desired sufficiently high gas-barrier properties have not been obtained. Then, as a method for improving such defects, an attempt to further form a gas-barrier layer on an inorganic thin film has been made. As a gas-barrier film obtained by such a method, a gas-barrier film in which an inorganic thin film has been coated with a resin layer containing an inorganic layered compound having a specific particle diameter and a specific aspect ratio has been disclosed (for example, Patent Document 1).
In addition, many films in which the surface of a plastic film has been coated with a resin composition having high gas-barrier properties have been proposed. As to resin compositions to be used for such films, a method that involves dispersing a flat inorganic substance such as an inorganic layered compound in a resin has also been known as a method for improving gas-barrier properties; for example, a product in which a barrier coating layer composed of an ethylene-vinyl alcohol-based copolymer, a water-soluble zirconium-based cross-linking agent, and an inorganic layered compound has been formed on a substrate film (for example, Patent Document 2) has been proposed.
Despite of the use of these methods, however, there have not been obtained gas-barrier films with which gas-barrier properties and lamination strength after retort treatment can be fully satisfied and which are stable in quality though improvement in characteristics under boiling or high humidity has been recognized.